The present invention relates to a postweld heat treatment process of carbon steel and low alloy steel, particularly to a postweld heat treatment process of carbon steel and ferrite-based heat resistant steel that is used at high temperature, which causes a problem of creep, for a long time.
Hitherto, in a welded joint portion where a weld metal or a welded heat affected zone is present, a drop in mechanical strengths such as tensile strength and creep strength has been caused and cracks have easily been generated and developed because of inhomogeneous texture generated by welding.
Conventionally, therefore, postweld heat treatment has been performed in order to reduce such a drop in mechanical strengths based on welding.
For example, in JIS Z3700 (postweld heat treatment process), the postweld heat treatment process of carbon steel and low alloy steel is defined. According to this standard, materials are divided to 6 kinds of P-1 to P-5 and P-9, and for each of the kinds a lowest holding temperature of postweld heat treatment is decided and further a shortest holding time, dependently on the thickness of weld zones, is also decided.
With respect to the temperature of the postweld heat treatment, It is prescribed that the upper limit of post heating temperature is not allowed to be over tempering temperature only about hardened and tempered steels made of the P-2 material and the P-9 material. On the other hand, about carbon steel and low alloy steel for which the upper limit of post heating temperature is not specified, it is prescribed that the upper limit of post heating temperature is usually set to a temperature 50xc2x0 C. lower than the Ac1 temperature of the materials.
For example, FIG. 10 shows stress-fracture time curves at 550xc2x0 C. about the base metal thereof (xcex94), weld metal (xe2x96xa1) and a welded joint (∘) subjected to a normal postweld heat treatment (PWHT) in the prior art, which consist of 2.25Cr-1Mo steel (JIS SCMV4NT). This figure demonstrates that the creep strength of the welded joint subjected to the nomal postweld heat treatment is lower than that of the base metal at the side of short times.
In conventional postweld heat treatment, steel is held at 690xc2x0 C. for 28 hours and cooled at a rate of 50xc2x0 C. per hour (50xc2x0 C./hour) or less. The long-term creep strength of a welded joint subjected to the conventional postweld heat treatment is not sufficiently improved, and damage based on the generation or development of cracks is not sufficiently suppressed.
On the other hand, the inventors recently made it evident that annealed structure generated by slow cooing from austenite temperature range has larger long-term creep strength than structure wherein martensite transformation or bainite transformation is generated.
Results of the examined mechanical strengths such as creep strength will be detailed hereinafter.
First of all, the following will describe an example in the prior art wherein 0.5Cr-0.5Mo steel was subjected to heat treatment to change initial texture, thereby yielding ferrite/perlite texture. According to this example, the following will be understood.
FIGS. 7 and 8 are graphs showing the effect of initial textures on long-term creep strength, and demonstrate that in order to obtain materials having high long-term creep strength, it is important to cool the materials slowly at a cooling rate lower than that of air-cooling from austenite single-phase temperature range.
The initial textures are different on the basis of difference in cooling rate from austenite single-phase temperature range. The cooling rate for martensite is largest, and that for bainite is middle. The cooling rate for ferrite plus perlite is lowest. Actually, ferrite plus perlite is subjected to furnace cooling.
The shown two kinds of tempered martensites are subjected to quenching followed by tempering treatment in the same way as in the martensite shown at the highest position.
FIG. 7 shows stress-fracture time curves of 0.5Cr-0.5Mo steels, which were obtained by changing the initial texture thereof by heat treatments. From this graph, it can be understood that within the long-term range of several tens of thousands of hours the annealed ferrite texture (black, diamond-shaped points: ♦) has a higher creep strength than the martensite texture (white round points ∘) and the bainite texture (white square points: xe2x96xa1), and also demonstrates that as time becomes longer, the annealed ferrite texture (black, diamond-shaped points) has a far higher creep strength than the martensite texture and the bainite texture.
FIG. 8 shows creep curves representing relationship between strain and time when the above-mentioned steels were kept at a temperature of 575xc2x0 C. and at a constant stress of 3.2 kgf/mm2. From this graph, it can be understood that the bainite texture (thick sold line) has a higher creep strength than the martensite texture (slightly thick sold line) but the annealed ferrite/perlite texture (thick dotted line) has a higher creep strength than the bainite texture.
As can be understood from the above, FIGS. 7 and 8 demonstrate that ferrite plus perlite (black, diamond-shaped points in FIG. 7) has the largest creep strength within the range of long times.
It can be therefore understood that ferrite plus perlite, for which cooling rate is small, has a larger long-term creep strength than martensite and bainite, for which cooling rate is large.
In turn, prepared were three kinds of 2.25Cr-1Mo steels whose initial textures were made different by different heat treatments, that is, JIS-STBA24 (annealing) JIS-SCMV4NT (normalizing, and tempering) and ASMEA542 (hardening, and tempering) Stress-fracture time curves of these steels at 550xc2x0 C. are shown in FIG. 9.
In this graph (FIG. 9), the cooling rate from austenite single-phase temperature range is smallest for the annealing (∘), is middle for the normalizing and tempering (xcex94) and is largest for the hardening and tempering (xe2x96xa1).
According to this graph, the following can be observed: within the high-stress and short-time area the hardened and tempered material (white round points) has the highest creep strength, but within the time range of several tens of hundreds of hours, in which the stress is 100 MPa or less, difference in creep strength among the three-kind steels becomes extinct and within the range of longer times the annealed material has a slightly higher creep strength. Therefore, in the case that use of the steels in an actual plant for a long time is considered, low short-term creep strength generated by slow cooling at a small cooling rate is not brought into a problem.
That is, a material cooled slowly at a cooling rate lower than that of air-cooling from austenite single-phase temperature range has a lower short-term creep strength than a quenched material. But, in the case that these materials are used in an actual plant for a long time of 10 years or more the effect of high strength based on the quenching becomes extinct. Therefore, in the case that use of the materials in an actual plant is considered, a drop in short-term creep strength generated by slow cooling at a cooling rate lower than that of air-cooling is not brought into a problem.
However, the conventional heat treatment is performed also to the steel materials having an excellent long-term creep strength. And, there remains a problem that in a welded joint portion where a weld metal or a welded heat affected zone is present, a drop in mechanical strengths such as tensile strength and creep strength is still caused and cracks are easily generated and developed because of inhomogeneous texture generated by welding.
In light of the above-mentioned situations, an object of the present invention is to provide a postweld heat treatment process which is more effective particularly for an improvement in long-term creep strength and suppression of damage based on the generation or development of cracks than conventional postweld heat treatment processes.
In order to solve the above-mentioned problems, according to a first aspect of the present invention, there is provided a postweld heat treatment process comprising the steps of holding a welded joint made of carbon steel or low alloy steel within austenite single-phase temperature range for a given time, thereby annealing the joint, and subsequently cooling the joint by air-cooling or by slow cooling at a cooling rate lower than that of the air-cooling.
According to a second aspect of the present invention, considering annealing temperature, a cooling means and cooling temperature, there is provided a postweld heat treatment process wherein the joint is held at 930xc2x0 C. for 30 minutes and is cooled by furnace cooling. According to a third aspect of the present invention, there is provided a postweld heat treatment process wherein the cooling rate upon the furnace cooling is set to 1xc2x0 C. per minute.
According to the present invention, by performing annealing (slow cooling from temperature Ac3 or more in austenite temperature range) as postweld heat treatment, the long-term creep strength of the welded joint is improved. Moreover, by causing inhomogeneous texture generated by the welding to extinct, it is possible to suppress essentially damage based on the generation or development of cracks resulting from the inhomogeneous texture.
About many high-temperature buildings such as boilers/turbines for power generation, atomic power generation facilities and plants for petroleum industry or petrochemical industry, the life spans thereof are controlled by damage of their welded joint portions. Therefore, by the postweld heat treatment of the present invention, it is possible that the safety and reliability thereof are improved and further the life spans thereof are prolonged.